1. Technical Field
The present application relates generally to a method for controlling tracking position during low-speed reproduction of video signals in a video cassette recorder (VCR) and, more particularly, to a method for automatically controlling the tracking position in accordance with a voltage level of an envelope waveform when reproducing at speeds lower than a normal reproduction speed of the VCR.
Additionally, the present method also prevents noise from being generated in a reproduced screen when reproducing a continuous moving image at low speeds to provide an optimally reproduced screen.
2. Description of the Related Art
In general, a conventional video cassette recorder (VCR) is a device which is capable of recording video and audio signals on a recording medium such as a video cassette tape and reproducing these signals from the recording medium. The VCR can reproduce the recorded video signal at various speeds, e.g., normal, low or high speed, or nearly stop reproducing the signal (i.e., display a paused image signal).
Generally, a conventional VCR includes a servo system which is divided into a drum servo for controlling the rotation of a head, and a capstan servo for controlling tracking and maintaining the running speed of the recording medium. Specifically, the capstan servo controls the phase and speed of a capstan motor so that the head which is allocated to each channel can accurately reproduce the video signal of the corresponding channel. The capstan servo also regularly maintains the running speed of the recording medium during recording or reproducing. The rotation speed of the capstan motor is controlled by obtaining a capstan speed error voltage from a capstan frequency generator (CFG) signal corresponding to the running speed of the recording medium. Therefore, during a particular reproducing process such as low-speed reproducing, a temporary stop, or a classifying operation, a step slow signal (i.e., frame-by-frame playback) is outputted from the microcomputer and applied to the capstan motor.
The phase of the capstan motor is controlled by adding the capstan speed error control voltage to a capstan phase error voltage which is obtained by comparing the frequency of a control signal with a reference frequency during the reproduction process.
In a conventional 2-head VCR, a first head (channel 1) and a second head (channel 2) are arrayed at the periphery of a head drum, and face each other 180 degrees apart. In order to enhance the recording density on recording medium, a method for obliquely recording the video signal on the recording medium per unit field is utilized. With this method, the video signal is alternatively recorded in a first field (field 1) corresponding to channel 1 and in a second field (field 2) corresponding to channel 2. The signals in field 1 and field 2 are recorded using an Azimuth recording method in order to prevent the signals from disturbing each other.
When the recording medium is run at a predetermined speed by the capstan motor and the head drum is rotated at a predetermined speed by a spindle motor, the first head reproduces the video signal recorded in field 1 (channel 1) and the second head reproduces the video signal recorded in field 2 (channel 2) according to a head switching pulse (HSP) signal. A video signal of one frame is then reproduced by composing the signals that are recorded in fields 1 and fields 2.
In a conventional 4-head VCR, a first standard play (SP) head and a second SP head (which are the main heads) are arrayed at the outer periphery of the head drum and face each other 180 degrees apart. A first super long play (SLP) head and a second SLP head (which are referred to as sub-heads) are located with respect to each other as much as 180 degrees apart at positions which are separated from the second and first SP heads, respectively. The first and second SLP heads are utilized for variable-speed reproducing.
Referring to FIG. 1, a tracing relation between the field of the video signal recorded on the recording medium and the corresponding head when reproducing at normal speed is shown. FIGS. 2A, 2B and 2C illustrate the relationship between the HSP signal, a control signal and an envelope waveform, respectively, when reproducing at normal speed. In particular, the video signals recorded in field CH1 and field CH2 are reproduced by the heads SP1 and SP2, respectively, according to the HSP signal (shown in FIG. 2A) and the control signal (shown in FIG. 2B). As a result of the periodic HSP and control signals, the envelope waveform of the reproduced video signals for each field is stable and has a constant level (as shown in FIG. 2C).
In a conventional low-speed reproducing method, a stopped (paused) screen is continuously reproduced at irregular intervals by simply rotating the capstan motor with the slow step method. The problem with this method is that the reproduced screen is stopped once per a predetermined period, which causes noise and flickering screens.
To solve this problem, a method for reproducing a continuous moving image at a low speed in a 4-head VCR is disclosed by the present applicant. Referring to FIGS. 3A through 3E, the SP heads and SLP heads are alternatively operated in accordance with a pseudo head switching pulse signal corresponding to the least common multiple of the speed, which enhances the level of the envelope waveform and results in a continuously reproduced image.
In the conventional low-speed reproducing method, since the tracking position is controlled within one period of the HSP signal, it is nearly impossible to decide an optimum tracking position at low-speed reproducing, thereby resulting a reduction in the quality of the reproduced image.
The present application is directed to a method for controlling a tracking position during low-speed reproducing in a conventional 4-head VCR in order to obtain an optimally reproduced image.
In one aspect, in a VCR having a pair of main heads for reproducing video signals from a recording medium at a normal operating speed associated with said VCR, a method for controlling tracking position when reproducing at speeds lower than the normal speed comprises the steps of:
detecting a period of an envelope waveform of a video signal which is reproduced by the pair of main heads;
generating a pseudo head switching pulse (PHSP) signal in accordance with the detected period; and
tracing a tracking position at a position where a voltage level of the envelope waveform is the largest within one period of the PHSP signal.
Preferably, the period indicates the number of the head switching pulses corresponding to the least common multiple of the speed to be reproduced.
These and other objects, features and advantages of the present application will become apparent from the following detailed description of illustrative embodiments, which is to be read in connection with the accompanying drawings.